1. Field of the Invention
The present invention relates to a reflecting optical element constituting an optical system used in digital cameras, projectors, and copying machines, and the like, in particular, to a reflecting optical element which has a plurality of reflecting surfaces with a curvature.
2. Description of the Related Art
Various optical elements having reflecting surfaces with a curvature (reflecting optical elements) have been proposed, for example, in U.S. Pat. No. 4,755,217 and Japanese Patent Laid-Open No. 2-297516, where are disclosed or proposed optical prisms having reflecting surfaces with a curvature.
The optical prism disclosed in U.S. Pat. No. 4,755,217 is related to the eyepiece configuration in an observation optical system, which is arranged as illustrated in FIG. 8.
In this figure, a displaying light flux 215 incident on an optical prism 210 as a divergent light flux from an information display 211 is totally reflected at a plane 212 of the prism, then reflected on a concave surface 213 having a concave curvature, and incident as a nearly parallel flux light on the pupil 214 of an observer due to the effect of the concave surface 213.
In addition, the above-mentioned prism 210 is so arranged that it allows displayed-image observation as well as object-image recognition. In other words, the light flux 216 from an object is incident on an refracting surface 217 nearly parallel to the above-mentioned plane 212 to reach the concave surface 213. There is a semitransparent film or the like deposited on the concave surface 213, and the light flux 216 from the object, after passing through the concave surface 213, passes through the plane 212 and is incident on the pupil 214 of an observer. Thus, the observer can observe the overlapped images due to the light flux 216 from the object and the displaying light flux 215.
On the other hand, it is known that it is possible to construct an optical system in which aberration is sufficiently corrected by making an optical surface to be an asymmetrically, aspheric surface, on the basis of the concept of the reference axis introduced into a decentered optical system. For example, Japanese Patent Laid-Open No. 9-5650 discloses a relevant method of design, while Japanese Patent Laid-Open No. 8-292371 and Japanese Patent Laid-Open No. 8-292372 disclose relevantly designed examples.
Such a decentered system is referred to as an off-axial optical system. When the reference axis is assumed to lie along the beam of light passing through the image center and the pupil center, an off-axial optical system is defined as an optical system which includes such an off-axial surface that the normal to the optical surface, at the point of intersection of the optical surface and the reference axis, is not on the reference axis. In an off-axial system the reference axis takes a bent shape.
In such an off-axial optical system, the optical surface becomes decentered, and the eclipse does not occur even on a reflecting surface, and hence it is easy to construct an optical system utilizing a reflecting surface. In addition, an off-axial optical system is characterized by the facts that there is a relatively high freedom in optical path routing and that construction of an integral optical system is easy to make on the basis of an integral-forming technique for forming an optical surface, and the like.
Accordingly, it is possible to construct a reflecting optical element which is excellent in space efficiency, compact and latitudinous in shape.
As for the above-described kind of reflecting optical elements, recently the mold fabrication using a metal mold prevails to meet a demand for lower cost. Mainly, these elements are formed of such resin materials as polystyrene, polymethyl methacrylate, polycarbonate, and the like.
In these resin-made elements, however, optical-performance deterioration is caused by the temperature variation, since there are fairly large differences in thermal expansion coefficient between the resin and inorganic materials.
When there are used such materials small in thermal expansion coefficient as glass, metal, and the like, the optical performance deterioration due to temperature variation does not cause serious problems, whereas there are many restraints in fabrication. For example, when glass is used, a smooth shape as a whole having neither steps or edges is required to be formed, since the presence of either steps or edges causes cracking formation therefrom.
In order to prevent these inconveniences, Japanese Patent Laid-Open No. 8-122505 has made an attempt to eliminate the cracking formation occurring at the time of fabrication, by smoothly joining the two surfaces abutting along the boundary through an appropriate configuration of the joint-surface shapes of the joint portions, when an optical member (a reflecting surface) is fabricated integrally from a plurality of optical members (reflecting surfaces).
Generally, there is no guarantee that all optical members can be joined so smoothly. Since a design permitting such smooth joining of optical members is necessarily accompanied by increasing restraints in fabrication, a desired optical performance is not easily achieved.